This invention relates to a length measuring device for generating a detection interference wave according to the distance between a light emitting portion, which is operative to emit laser light, and a moving detection portion, for A/D-converting the aforesaid detection interference wave and a reference interference wave to thereby obtain the phase difference between both of the interference waves, and for determining the position of the aforementioned moving detection portion from this phase difference and the detected wavelength of the aforesaid laser light.
A conventional length measuring device will be described hereinbelow with reference to FIG. 4, FIG. 5, FIG. 6. FIG. 7 and FIG. 8. FIG. 4 is a block diagram schematically showing the configuration of a conventional length measuring device. Further, FIG. 5 is a diagram showing a concrete length measuring mechanism of the conventional length measuring device. Moreover, FIG. 6 is a diagram showing a part of a signal processing portion (namely, a phase difference processing portion), an operation portion and a storage portion of the conventional length measuring device. Moreover, FIG. 7 is a diagram illustrating the relation between the wavelength of laser light thereof and the ratio of the intensity of transmitted light to that of reflected light thereof. Moreover, FIG. 8 is a diagram showing the configurations of another part of the signal processing portion, the operation portion and the storage portion of the conventional length device.
In FIG. 4, reference numeral 100 designates a light emitting portion for emitting laser light having a frequency f; 200 an interference system; 300 a moving detection portion; 400 a light receive portion; 500 a signal processing portion; 600 an operation portion; and 700 a storage portion.
In FIG. 5, reference numeral 100 denotes a laser diode (LD) composing the light emitting portion; and 201 an acousto-optic modulator (AOM) adapted to generate light, whose frequency is (f+f1), when receiving laser light, whose frequency is f, from the laser diode 100. Similarly, reference numeral 202 represents an acousto-optic modulator (AOM) which is driven at a frequency f2 and generates light, whose frequency is (f+f2), when receiving laser light, whose frequency is f, from the laser diode 100. The frequency difference between the frequency f1 and the frequency f2 is set at a very small value.
Light 801 outputted from the acousto-optic modulator 201 is reflected by a mirror 301 of the moving detection portion 200 and is then incident on the light receiving portion 400 through the interference system 200. Further, light outputted from the acousto-optic modulator 202 is deflected by a prism 203 by a very small angle and thus becomes light 802 which is then reflected by the mirror 301 of the moving detection portion 300 and is further incident on the light receiving portion 400 through the interference system 200. Incidentally, reference numeral 204 designates a beam splitter of the wavelength-dependent type; 805 reflected light; and 806 transmitted light.
Further, in FIG. 5, reference numeral 401 denotes an optical element (namely, PD: photo diode), which is provided in the light receiving portion 400 as shown in this figure. This optical element 401 is operative to detect an interference wave 803 formed from the interference between light coming from the acousto-optic modulator 201 and light coming from the acousto-optic modulator 202. Similarly, reference numeral 402 designates an optical element (namely, PD: photo diode). This photo diode 402 is operative to detect an interference wave 804 produced from the interference between the light 801 and the light 802 which are incident on the light receiving portion 400.
The interference wave 803 inputted to the light receiving element 401 is employed as a reference interference wave. The position of the moving detection portion 300 is determined on the basis of the phase difference between this reference interference wave and the detected interference wave 804 which is inputted to the light receiving element 402.
As illustrated in FIG. 6, an output of the light receiving element 401 is inputted to a current-to-voltage conversion circuit 501 and is then converted into a voltage therein. Similarly, an output of the light receiving element 402 is inputted to a current-to-voltage conversion circuit 502 and is then converted into a voltage therein. A phase-difference count circuit 504 is operative to count clocks outputted from a phase-difference count clock generating circuit 503 in a time period between a zero-cross point of an output waveform of the current-to-voltage conversion circuit 501 and a zero-cross point of an output waveform of the current-to-voltage conversion circuit 502. A result of the counting performed by this phase-difference count circuit 504 is inputted to the microcomputer 600 composing the operation portion. This microcomputer 600 is operative to obtain a phase difference, which is represented by using an electrical angle, according to the result of the counting.
When the difference in optical path length between the two interference waves changes by a wavelength of the laser light, the phase difference therebetween varies by 360. Therefore, a quantity acquired by adding 2xcfx80 n (incidentally, xe2x80x9cnxe2x80x9d is an integer) to the phase difference obtained in the aforementioned manner is a total phase difference. The position of the moving detection portion 300 is determined from this total phase difference. The integer xe2x80x9cnxe2x80x9d is determined by counting cycles, which correspond to the time duration of the interference wave outputted from the light receiving element 402 while the moving detection portion 300 from an origin to a current position thereof, by means of the phase-difference count circuit 504.
Generally, the wavelength of laser light is liable to vary. Moreover, the phase-difference between the interference waves is dependent on the wavelength of laser light outputted from the light emitting portion (namely, the laser diode) 100. Thus, the aforementioned distance cannot be known only by obtaining the total phase difference between the interference waves. It is, therefore, necessary to know the exact wavelength of the laser light. In the case of the conventional length measuring device, the wavelength of laser light is detected from transmitted light and reflected light, into which the laser light is split by the beam splitter 204 of the wavelength-dependent type, in the following manner.
As shown in FIG. 5, the light, which has a wavelength xcex and is incident on the beam splitter 204 of the wavelength-dependent type, is split into the transmitted light 806 and the reflected light 805. Furthermore, as illustrated in FIG. 7, there is established a predetermined relation between the wavelength xcex and (the ratio of the intensity of the transmitted light to the intensity of the reflected light). Therefore, the wavelength xcex of the incident light (namely, the laser light) can be found if the ratio of the intensity of the transmitted light to the intensity of the reflected light is known.
As illustrated in FIG. 8, the reflected light 805 coming from the beam splitter 204 of the wavelength-dependent type is incident on a light receiving element 403 and is then converted into a voltage by the current-to-voltage conversion circuit 505. This voltage is converted by an A/D converter 511 through a sample-and-hold circuit 507 and a multiplexer 508 into digital data which is subsequently supplied to the microcomputer 600.
Similarly, the transmitted light 806 of the wavelength-dependent type beam splitter 204 is incident on the light receiving element 404 and is then converted into a voltage by a current-to-voltage conversion circuit 506. This voltage is converted by the A/D converter 511 through the sample-and-hold circuit 507 and the multiplexer 508 into digital data which is subsequently supplied to the microcomputer 600.
The microcomputer 600 determines (the ratio of the intensity of the transmitted light to the intensity of the reflected light) on the basis of these digital data, and further obtains a wavelength xcex1 from the relation between (the ratio of the intensity of the transmitted light to the intensity of the reflected light) and the wavelength xcex, which is illustrated in FIG. 7. Incidentally, the upper limit value and the lower limit value of the reference voltage value of the A/D converter 511 are generated by a reference voltage upper-limit-value generating circuit 509 and a reference voltage lower-limit-value generating circuit 510, respectively.
The aforementioned conventional length measuring device has a problem in that when the aforesaid distance is determined by detecting the phase difference between the interference wave signals, it is necessary for enhancing phase-difference detecting resolution to increase the clock frequency of the phase-difference count clock generating circuit 503 which is used for detecting the aforementioned phase difference illustrated in FIG. 6, and this is technically difficult to achieve. For example, there has been the need for further increasing the clock frequency which is usually 200 MHz or so. This has been very difficult to achieve.
Further, the conventional length measuring device has another problem in that although the resolution used at the time of A/D-converting the reflected light 805 and the transmitted light 806, which come from the beam splitter of the wavelength-dependent type 204, by means of the A/D converter 511 is enhanced by increasing the resolution used by the A/D converter 511, countermeasures against variation in data due to noises are required and a filter is needed.
Moreover, the conventional length measuring device has still another problem in that in the case where a reference (namely, a reference voltage) of the A/D converter 511 varies owing to a change in power supply, an error occurs in A/D-converted data. For instance, in the case of a 12-bit A/D converter having such a voltage of 5 V, the variation in voltage corresponding to data outputted thereof is 1.2 mV/bit. In contrast, in the case of an ordinary power supply, a variation in power supply voltage thereof is 10 mV or so. Moreover, a power supply, which assures a supply voltage at a smaller variation, is expensive.
Furthermore, the conventional length measuring device has yet another problem in that precise temperature control, or accurate control over an injection current to the laser diode 100 is needed so as to prevent the wavelength of laser light coming from a light source from varying, and this results in increase in complexity and cost of the device.
Additionally, the conventional length measuring device has a further problem in that the use of an expensive high-resolution A/D converter is required so as to accurately detect a variation corresponding to a wavelength of laser light by using the beam splitter 204 of the wavelength-dependent type.
Besides, the conventional length measuring device has another problem in that an error due to drift of an electronic circuit is included in A/D-converted data obtained by A/D-converting the reflected light 805 and the transmitted light 806, which come from the beam splitter 204 of the wavelength-dependent type, and thus an error is caused in the detecting accuracy thereof.
Further, the conventional length measuring device has still another problem in that a variation in the correlation characteristics between the ratio of the intensity of the transmitted light 806 to the intensity of the reflected light 805 and the wavelength is caused in the beam splitter 204 of the wavelength-dependent type under the influence of a change in temperature, and consequently, an error occurs in the detected wavelength.
This invention is accomplished to resolve the aforementioned problems. Accordingly, an object of this invention is to obtain a length measuring device which is operative to generate a sinusoidal wave signal and a cosinusoidal wave signal from an interference wave signal by differentiation or integration, and then A/D-converting these sinusoidal and cosinusoidal wave signals, and subsequently, calculate an electrical angle by electrical interpolation, and moreover, calculate another electrical angle from another interference wave signal in a similar manner, and then calculate a phase difference between both of the electrical angles, and furthermore, correct an error caused by an electronic signal, and which thereby can detect an accurate position of the moving detection portion at all times.
Further, another object of this invention is to obtain a length measuring device which is operative to hold a sinusoidal wave signal, a cosinusoidal wave signal, the inverting signals obtained by inverting these signals, and a reference voltage (namely, a standard voltage) simultaneously, and which doubles the number of divisions for the A/D-converted data without enhancing the resolution used by the A/D converter, and which thus can achieve the high-resolution detection of the position of the moving detection portion in a stable manner even when a change in the power supply occurs.
Moreover, still another object of this invention is to obtain a length measuring device which can achieve the high-precision detection of the wavelength of laser light and the accurate detection of the position of the moving detection portion without using a high-resolution A/D converter by varying the reference voltage of an amplifier for amplifying the transmitted light and the reflected light coming from a beam splitter of the wavelength-dependent type in the case that the variation in laser wavelength is detected by the aforesaid beam splitter of the wavelength-dependent type.
Furthermore, yet another object of the present invention is to obtain a length measuring device which can achieve the accurate detection of the position of the moving detection portion by interrupting electric current outputted from the light receiving element and correcting a variation corresponding to a drift caused by the electronic circuit.
Additionally, a further object of this invention is to obtain a length measuring device which can achieve the accurate detection of the position of the moving detection portion without high-precision temperature control and without injection current control, by being provided with a fixed detecting portion, which is placed at a constant distance from the light emitting portion at all times, independent of the distance between the moving detection portion, which is adapted to move, and the light emitting portion.
A length measuring device of the present invention comprises: a light emitting portion for emitting laser light; an interference system for receiving the aforesaid laser light, for generating reference light having a first frequency, for outputting the reference light straight, for generating detection light having a second frequency, and for outputting the aforesaid detection light at a predetermined tilt angle; a moving detection portion for reflecting the aforesaid reference light and the aforesaid detection light toward the aforesaid interference system; a light receiving portion for converting a reference interference wave, which is generated by the aforesaid interference system, into a first sinusoidal wave electrical signal according to the aforesaid laser light and for converting a detection interference wave, which is generated from an optical-path-length difference between the aforesaid reflected reference light and the aforesaid reflected detection light, into a second sinusoidal wave electrical signal; a signal processing portion for generating a first cosinusoidal wave signal from the aforesaid first sinusoidal wave signal, for generating a second cosinusoidal wave signal from the aforesaid second sinusoidal wave signal, and for A/D-converting the aforesaid first sinusoidal wave signal and the aforesaid first cosinusoidal wave signal, and the aforesaid second sinusoidal wave signal and the aforesaid second cosinusoidal wave signal; and an operation portion for obtaining an electrical angle of the aforesaid reference interference wave according to a ratio between the A/D-converted first sinusoidal wave data and the A/D-converted first cosinusoidal wave data, for obtaining an electrical angle of the aforesaid detection interference wave according to a ratio between the A/D-converted second sinusoidal wave data and the A/D-converted second cosinusoidal wave data, for obtaining a phase difference between the aforesaid reference interference wave and the aforesaid detection interference wave from the aforesaid two electrical angles, and for detecting a position of the aforesaid moving detection portion according to this phase difference and a wavelength of the aforesaid laser light.
Further, a length measuring device of this invention is adapted so that the aforesaid signal processing portion includes a cosinusoidal wave generating circuit for generating a cosinusoidal wave signal, and that the aforesaid operation portion is operative to correct a phase error of the aforesaid cosinusoidal wave signal generating circuit according to an addition theorem applied to trigonometric functions.
Moreover, a length measuring device of this invention is adapted so that the aforesaid cosinusoidal wave generating circuit is a differentiating circuit, and that the aforesaid operation portion is operative to correct a phase lag of the aforesaid differentiating circuit according to an addition theorem applied to trigonometric functions.
Additionally, a length measuring device of this invention is be adapted so that the aforesaid cosinusoidal wave generating circuit is an integrating circuit, and that the aforesaid operation portion is operative to correct a phase advance of the aforesaid integrating circuit according to an addition theorem applied to trigonometric functions.
Besides, a length measuring device of this invention is adapted so that the aforesaid signal processing portion includes an inverting circuit, a sample-and-hold circuit and an A/D converter, and that the aforesaid inverting circuit is operative to perform an inversion on the aforesaid first sinusoidal wave signal and the aforesaid first cosinusoidal wave signal and the aforesaid second sinusoidal wave signal and the aforesaid second cosinusoidal wave signal, that the aforesaid sample-and-hold circuit holds the aforesaid inverting signals, which are obtained by the inversion, and original non-inverting signals simultaneously, and that the aforesaid A/D converter is operative to A/D-convert the aforesaid signals held simultaneously.
Further, a length measuring device of the present invention is adapted so that the aforesaid signal processing portion includes a reference voltage generating circuit for generating a reference voltage of the aforesaid A/D converter, and that the aforesaid sample-and-hold circuit is operative to simultaneously hold the reference voltage of the aforesaid A/D converter when simultaneously holding the aforesaid inverting signal and the aforesaid original non-inverting signal.
Moreover, a length measuring device of the present invention is adapted so that the aforesaid system includes a beam splitter of the wavelength-dependent type for receiving the aforesaid laser light and for outputting reflected light and transmitted light, which are respectively obtained by reflecting and transmitting the aforesaid laser light, that the aforesaid light receiving portion converts the aforesaid reflected light and the aforesaid transmitted light into electrical signals, and that the aforesaid signal processing portion includes an amplifying circuit for amplifying the aforesaid electrical signals, and a reference voltage generating circuit for generating a reference voltage of the aforesaid amplifying circuit, and that the aforesaid operation portion is operative to change the reference voltage of the aforesaid amplifying circuit by controlling the aforesaid reference voltage generating circuit according to variations in amounts of the aforesaid reflected light and the aforesaid transmitted light, which are caused owing to a change in wavelength of the aforesaid laser light.
Furthermore, a length measuring device of this invention is adapted so that the aforesaid signal processing portion includes a switch for turning on or off an electrical signal coming from the aforesaid light receiving portion, and that the aforesaid operation portion is operative to eliminate a variation, which corresponds to a drift, by subtracting data, which is obtained when the aforesaid switch is turned off, from data obtained when the aforesaid switch is turned on.
Additionally, a length measuring device of this invention further comprise a fixed detection portion, which has a structure similar to that of the aforesaid moving detection portion and is fixed at a known distance from the aforesaid interference system, and is adapted so that the aforesaid operation portion is operative to calculate a phase difference relating to the aforesaid fixed detection portion similarly as in a case of the aforesaid moving detection portion, and to correct the detected phase difference relating to the aforesaid moving detection portion.